1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens. For example, the image pickup apparatus is suitable for use as a video camcorder, an electronic still camera, a silver-halide film camera, or the like.
2. Description of the Related Art
In recent years, image pickup apparatuses, such as video camcorders, digital still cameras, broadcasting cameras, and silver-halide film cameras, using a solid-state image pickup device have become more sophisticated and compact. Accordingly, it is demanded that a compact zoom lens with a short overall length and high resolving power be included as a photographing optical system in such image pickup apparatuses.
As a zoom lens having a compact optical system and a high zoom ratio, there is known a four-unit zoom lens including, in order from the object side to the image side, first, second, third, and fourth lens units having positive, negative, positive, and positive refractive powers, respectively. This four-unit zoom lens performs zooming by moving each lens unit.
Among four-unit zoom lenses of this type, there is known a so-called rear focusing zoom lens which performs focusing by moving the fourth lens unit (see, e.g., U.S. Pat. Nos. 6,462,886 and 6,166,864).
For compact storage and better portability of a camera (i.e., image pickup apparatus), it is effective to use a retractable zoom lens in which lens units retract when the camera is not in use (i.e., when the camera is in non-photographing mode).
As a zoom lens that can meet such a need, there is known a zoom lens which is short in the overall length of each lens unit and suitable for use as a retractable zoom lens. This zoom lens includes, in order from the object side to the image side, first, second, third, and fourth lens units having positive, negative, positive, and positive refractive powers, respectively (see, e.g., U.S. Pat. Nos. 6,016,228, 6,456,441, 6,975,461, US Patent Application No. 2005/0007480, and U.S. Pat. No. 7,206,137).
At the same time, there is also a known zoom lens in which some of its lens units are displaced for correction of image blur. For example, in a four-unit zoom lens including, in order from the object side to the image side, first, second, third, and fourth lens units having positive, negative, positive, and positive refractive powers, respectively, the third lens unit is vibrated in a direction perpendicular to an optical axis so as to form a still image (see, e.g., U.S. Pat. No. 5,585,966).
Generally, a compact zoom lens can be realized by reducing the number of lens elements constituting the entire zoom lens while increasing the refractive power of each of lens units constituting the zoom lens. However, in a zoom lens constructed as described above, an increase in the refractive power of each lens surface means an increased lens thickness. This not only causes an insufficient reduction in the overall length of the lens system, but also makes it difficult for the zoom lens to correct aberrations and provide high optical performance.
Therefore, to realize a compact zoom lens with a high zoom ratio and high optical performance, it is necessary to appropriately define movement of each lens unit for zooming, refractive power of each lens unit, and lens configuration in each lens unit.
At the same time, due to the mechanical structure of a camera, retraction and storage of lens units while the camera is not in use cause an increase in error resulting from the tilt of lens elements and lens units. In this case, if the degree of sensitivity which may affect the optical performance of the lens elements and lens units is high, degradation in optical performance may result or image jitter may occur during zooming. Therefore, to achieve high optical performance, it is desirable to configure the zoom lens such that the sensitivity of the lens elements and lens units is minimized.
As described above, it is important for a zoom lens to achieve both compactness of the entire optical system and high optical performance.